1. Field of the Invention
This invention pertains generally to receiving digital radio frequency communications signals, and more particularly to extracting a signal of interest from noise and interfering multipath signals.
2. Description of the Background Art
The problem of extracting a signal from a noisy environment is well known in the signal processing arts. In a typical high frequency (HF) environment, phase shifted multipath and Doppler spread interference can result from ionospheric reflections. In addition, reflections from obstructions in the path of the signal can cause reflections of signals at other frequencies. The fundamental problem facing any receiver designer is how to improve the reception of a signal of interest in the presence of unknown and undesired interfering signals. In principle, this can be accomplished by signal processing.
Where data streams are involved, decoders employing the Viterbi algorithm have heretofore been considered to be the optimum method for detection of a data sequence in the presence of intersymbol interference and additive Gaussian noise. However, the computational complexity of Viterbi decoders is very large--approximately P.sup.L, where P is the size of the signal set and L is the length of the channel impulse response (CIR). For this reason several simplifications have been developed, most of which are more effective when dealing with a minimum phase CIR. The simple decision feedback equalizer (DFE) decoder is also more efficient in the case of a minimum phase CIR.
In a typical HF environment, however, the combined effect of multipath and Doppler spread often causes the channel to have a non-minimum phase impulse response. A possible solution is to use a pre-equalizer before the decoder to get a shorter or minimum phase desired impulse response (DIR). This solution can have one or two types of disadvantages, however. First, the zeros of the CIR need to be computed and, second, the pre-equalizer could enhance the noise level.
Therefore, there is a need for a non-complex and easily implemented apparatus and method for extracting a signal of interest from interfering multipath and. Doppler spread signals which does not result in an unacceptable increase in noise. The present invention satisfies that need, as well as others, and overcomes deficiencies and limitations of prior methods and devices.